The use of 4-phenylazopyrazolone masking couplers is known in the art. See ,for example, U.S. Pat. Nos. 2,428,034; 2,434,272; 2,455,170; 2,688,539; 2,704,711; 2,808,329; 3,476,560; 3,796,574; 4,427,763; 4,777,123, and EP 213,490; as well as those identified in Research Disclosure December 1989, Section VII, Part G, Publiched by Kenneth Mason Publications, Ltd., Dudley Annex, 12A North Street, Emworth, Hampshire PO10 7DQ, England. These compounds have proven useful since they are yellow colored in nonexposed areas and magenta colored in exposed areas. Thus, when in reality the magenta dye formed in a color negative photographic process has a small but significant unwanted absorption in the blue range, this may be balanced somewhat by the relative loss of blue absorption due to conversion of the mask color from yellow to magenta in the exposed areas. Then, an adjustment can be made to the spectral content of the light used to produce the positive from the negative to effectively cancel out the unwanted blue absorption which is now relatively constant across both the exposed and unexposed areas of the negative.
While phenylazopyrazolone masking couplers have been employed as a means of offsetting the unwanted blue absorption of conventional magenta couplers, this means for improving the color rendition has now been found to be responsible for increased fogging of the photographic element during processing. This is thought to be due to the formation of a phenyldinitrogen species from the masking coupler during development and/or the presence of undesired reducing agents. This in turn causes the unwanted nonimagewise development of the photographic silver halide contained in the photographic element.
European Patent Application 232,101 discloses a photographic element containing a pyrazolotriazole coupler together with at least 17 mole % of a colored masking coupler that may be of the azopyrazolone type. The presence of the large relative percentage of the masking coupler is said to improve sharpness and grain. There is no suggestion of the advantages to be obtained by including a ballasted aromatic nitro compound and, in fact, the higher concentration of masking coupler suggested would serve to aggravate the fogging problems. U.S. Pat. No. 4,777,123 contains similar general disclosure but again does not suggest the advantage of using the ballasted aromatic nitro compound. U.S. Pat. No. 4,600,688 proposes broad combinations of pyrazolotriazoles and pyrazolones as image couplers having an advantageous color absorption spectrum, but no ballasted aromatic nitro compounds are suggested.
U.S. Pat. No. 4,132,551 describes nitroaromatic compounds useful in photographic elements as antifoggants. The mentioned materials appear to act on the silver of the photographic emulsion to prevent fogging during development at elevated temperature. Nitrobenzene compounds are suggested generally as antifoggants perhaps in combination with azopyrazolones in U.S. Pat. Nos. 4,277,559; 4,977,072; 4,163,670; U.K. Specification 1,269,268; and Research Disclosure 17643 Section VI-I (1975). It is noted however that these suggestions are related to the inclusion of such compounds as general antifoggants in a photographic emulsion where they are believed to adsorb to the silver halide grain surface and prevent nonimagewise reduction of the silver. There is no recognition of the significance of locating the compounds of this invention in the same layer as an azopyrazolone masking coupler to efficiently trap any phenyldinitrogen species that might be formed from the masking coupler and wander toward the photographic silver.
It would be desirable to provide a photographic element and process where an azopyrazolone masking coupler can be used without incurring increased fogging during development and where this can be accomplished without sacrificing other photographic properties such as speed.